Background: Non-standardized data collection in medical records has a negative impact on the quality of care: it increases the possibility of medical errors, hinders the performance of clinical studies, precludes the development of improvement plans and impedes accreditation in quality-of-care programs. The ASCO QOPI (Quality Oncology Practice Initiative) certification program accredits the quality of care of oncology departments through a self-assessment process. An overall quality score (OQS) above 75% on 26 measures that focus on quality of care and patient safety is required for application. In order to begin the certification process, we analysed the quality of the data collected in our initial medical history to implement improvement actions. Methods: Based on the 26 QOPI measures, plus 17 additional measures that were chosen by consensus as relevant, we performed a retrospective baseline analysis of the data collected in the initial medical history of patients assessed in June 2021. Patients with breast cancer, colorectal cancer and non-small cell lung carcinomas who initiated systemic treatment were included. Those who participated in clinical trials were excluded. The OQS of the 26 patients analyzed was 59%. Given this negative result, and once the problem was identified, the objective was established (OQS > 75%), changes were prioritized and an improvement plan was developed based on rounds of training, feedback and reinforcement to the entire medical team, and through the design of a template with the essential data to be collected. Results: With the baseline result of June 2021 (n = 26; OQS = 59%), improvement rounds were performed in November 2021 (n = 29; OQS = 82%), December 2021 (n = 26; OQS = 74%), January 2002 (n = 35; OQS = 75%), February 2022 (n = 32; OQS = 78%) and March 2022 (n = 31; OQS = 79%). Conclusions: Correct data collection in medical records is essential for quality of care. Simple short-term measures based on training, feedback and reinforcement, and the creation of a data template, have increased OQS from 59% to exceed the 75% threshold required by QOPI to apply for the certification program. However, a long-term sustainable plan needs to be established through the introduction of computerized checklists in the free-text electronic medical history.